This invention relates to a lubricant additive which imparts enhanced rust and corrosion resistance to engine lubricating oils, and to a lubricating oil composition containing such an additive. More specifically, this invention relates to a marine diesel engine lubricant additive comprising a polythiocyclic imide and a poly[(ethylene-co-propylene-g-succinic anhydride].
The primary function of lubricants is to decrease friction. Frequently, however, lubricating oils need additional properties, such as oxidation and corrosion resistance, to be used effectively. For example, lubricants used in the crankcases of large diesel engines, such as marine and railway diesel engines, are often subjected to operating conditions requiring special considerations. In particular, poor grade fuels, such as marine residual fuel, can be mixed with regular diesel fuel, such as D-2, for fuel cost savings. However, engine performance problems, such as increased corrosion and poor oxidative stability, often arise. Additionally, new and more fuel efficient railway diesel engines place greater mechanical and chemical demands on the oxidation and corrosion resistance of lubricants. Lubricants used for such newer engines are typically changed more frequently or reformulated to limit corrosion, since, once oxidized, lubricants cause increased engine corrosion.
Various additives have been used to improve the rust and corrosion inhibition of lubricants. For example, calcium sulfurized alkyl phenolates have been effective oxidation and corrosion inhibitors. These compounds are typically made by processes including expensive recovery procedures and hazardous byproducts which are difficult to dispose of in an environmentally sound way. Alternatively, calcium sulfurized phenolates, made by a process using lime as the calcium source to avoid such processing problems, fail to provide a high level of rust and corrosion inhibition.
Lubricants used in the crankcase of a slow speed crossheaded marine engine must protect the engine parts from corrosion and especially rust. Rust is produced when ferrous metal engine components come in aliphatic with water; typically through two avenues. The first involve the internal combustion process where water is produced; the second from outside sources, that is, fresh water or saline water. In both cases, however, the net effect is identical: rust and corrosion that reduce engine efficiency and lifetime.
Rust and corrosion inhibitors are added to lubricating oils as part of the overall additive package comprising between 0.5 weight percent to perhaps as much as 2.5 weight percent. Ideally, rust and corrosion additives should be completely miscible with lubricating oils. Moreover, it is essential that these materials are completely compatible with other additives therein the lubricating oil. Rust or corrosion inhibitor additives that interact with other lubricating components are of little concern unless an aesthetically unappealing appearance results. This incompatibility with either the lubricating oil itself or additives therein is generally manifested by haze or solution turbidity. Despite the identical performance of both turbid and clear lubricating oil blends, consumer-perceived product imperfection will mar successful marketing of the oil blends.
Various additives have been used to improve the oxidation and corrosion resistance of lubricants. For example, calcium sulfurized alkyl phenolates have been effective oxidation and corrosion inhibitors. These compounds are typically made by processes including expensive recovery procedures and hazardous by products which are difficult to dispose of in an environmentally sound way. Alternatively, calcium sulfurized phenolates, made by a process using lime as the calcium source to avoid such processing problems, fail to provide a high level of oxidation and corrosion resistance.
Thus, there is a need for a lubricant additive which will provide effective oxidation and corrosion resistance, without posing the environmental hazards of other oxidation and corrosion inhibitors.
Therefore, it is an object of the present invention to provide both an additive composition and a lubricating oil composition which have significant oxidation and corrosion resistance.